1. Field of the Invention
The invention is generally related to nuclear fuel assemblies and particularly to the fabrication of spacer grids used in nuclear fuel assemblies.
2. General Background
Fuel assemblies for nuclear reactors are formed from a number of fuel rods held in position radially by spacer grids located along the length of the fuel rods. The spacer grids are formed from slotted metal strips containing stamped features, which are crisscrossed on edge to define individual cells for each fuel rod. The crossing metal strips are aligned at approximate right angles to each other and then welded together. The alignment method currently used is generally known as the pinning and framing process. After the metal strips are placed in the crisscross pattern, the next step is the insertion of pins into the individual cells of the egg crate. The "egg crate" is used to refer to the unwelded spacer grid inner structure comprised of intersecting, slotted strips that form a checkerboard-like array of cell locations. A tooling pin is then inserted into each cell. The pin interfaces with the base strip areas, as opposed to the contact points in each cell, to position and hold the strips in the proper orientation. The pinned egg crate is then placed into a welding frame that contains pressure pads which secure the egg crate to the frame. Pressure applied directly to the outer tooling pins is transferred to the inner locations through each strip and the adjacent pins. The load is repeatedly applied and released to act as a settling process to remove any tiny gaps or misalignments that may be present. The grid is then ready to be welded. This leaves a need for an improved fixturing system that can be used to align the egg crate with a minimum of steps and equipment, and provide greater dimensional control.